WASHINGTON – A senior Heritage Foundation employee who has publicly defended the think tank’s outgoing president was placed on administrative leave without explanation Monday afternoon.

Sources familiar with the situation told The Post that James Wallner, Heritage’s Vice President for Research, was asked to leave around noon Monday and to not return to the conservative think tank “until told otherwise.”

Wallner had been in severance talks with the Foundation but was given no warning that he would be asked to leave without reaching an agreement.

The former Senate staffer arrived at Heritage last summer and was swift to introduce changes to the think thank’s research structure and policy development, sources said.

Wallner blasted off a series of tweets over the weekend defending Heritage Foundation president Jim DeMint, who is expected to be formally dismissed by the think tank’s 22-member board of trustees on Tuesday. News of DeMint’s ouster spread like wildfire last week and has fueled competing narratives in the media.

Allies of the former South Carolina senator claim he was forced out by Mike Needham, the chief executive of political advocacy organization Heritage Action, who opposed the direction of the group under DeMint’s leadership.

“There are a lot of big opinions from people on Capitol Hill about Mike,” said a senior GOP source, who predicted that Wallner won’t be the only DeMint loyalist to exit Heritage this week.

“The next few days will be revealing,” the source said.

Needham has stayed mostly silent with the exception of an appearance last weekend on Fox News Sunday, during which he declined to strike down rumors that White House chief strategist Steve Bannon could take the helm of Heritage.

“That was floated to tamp down our small-dollar donor base who are furious about what’s happening to DeMint,” a Heritage source told The Post.

Heritage’s governing board is set to meet Tuesday morning to vote to dismiss DeMint. Ed Feulner, a co-founder of the think tank as well as its former president, has taken over as president in the interim.